Developing and evaluating implicit solvent models for catalytic metallic surfaces

Abstract

Understanding solvent effects at the metallic/liquid interface is critical to improving and analysing heterogeneous catalytic processes. In addition to a growing body of experimental work, computational studies are elucidating how the presence of water affects both the electronic structure and the adsorption thermodynamics of the metallic surface. However, computational methods such as ab initio Molecular Dynamics (AIMD) require extensive configurational sampling to obtain equilibrated thermodynamic quantities, precluding their use for wide-ranging studies. In contrast, by considering the dynamic degrees of freedom as an average, implicit solvent methods provide a route to tractable computational simulations of the aqueous environment, while maintaining a quantum mechanical description of metallic/adsorbate interactions. This thesis, in collaboration with the Pacific Northwest National Laboratory (PNNL), describes how implicit solvent approaches can be applied as an inexpensive method of evaluating both the electronic structure of the metal/liquid interface, and the free energy change of adsorption in the aqueous phase for a range of organic adsorbates. To ensure these calculations are performed accurately and efficiently, developments were made to the linear scaling Density Functional Theory (DFT) code, ONETEP as part of this work. These developments include the implementation of the soft sphere dielectric cavity model, which gives the flexibility to parameterize the solvent model for individual atomic centres. This contrasts with the original electron density based cavity model, which applies a global cavity parameter, leading to poor descriptions of the free energy changes of solvation for systems with mixtures of light organic and heavy metallic species. A surface accessible volume term was also implemented for the non-polar solvation term, which improves the correlation with experimental solvation free energies compared to the surface area non-polar term. Furthermore, a Pulay Hamiltonian mixing routine was implemented in the Ensemble DFT (EDFT) scheme of ONETEP. This approach confers significantly improved convergence behaviour for single point energy calculations performed in this work. This enables more efficient and accurate simulations of metallic systems, allowing for the evaluation of larger systems studied in later chapters. Utilising the implemented soft sphere model, this work assesses the ability of the soft sphere model to capture the potential of zero charge and the work function of the metallic/liquid interface. By reparameterizing the implicit solvent model in terms of the work function values calculated from snapshots of an AIMD simulation, we were able to capture the salient electronic structure changes of the solvated metallic surface and electrochemical properties. Then, combining the accelerated EDFT scheme and the implicit solvent parameterization method used for the potential of zero charge, this thesis concludes with a continuum solvent approach for calculating the aqueous phase adsorption free energy of organic molecules to the Pt(111) surface. In this work, approximations are derived for the entropies of solvation for the metallic surface based on analytical statistical thermodynamic expressions. These approximations allow us to parameterize the implicit solvent mode ∆G_solv for the metallic surface, enabling adsorption free energy with reasonable accuracy for a range of coverages and orientations. This opens a route for computationally inexpensive evaluations of adsorption processes at the aqueous Pt(111) interface, which can provide an atomistic understanding of adsorption processes in support of experimental studies. The work presented in this thesis shows the usefulness of the implicit solvent method in studies of heterogeneous catalytic processes and electrochemical interfaces. The techniques described in this work show that thermodynamic and electrochemical properties can be calculated in a computationally tractable manner with implicit solvent. In future, this could enable high throughput studies for a range of metallic surfaces and adsorbates, aiding the design of catalysts for a range of applications.

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ORCID for Chris-Kriton Skylaris: orcid.org/0000-0003-0258-3433

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